3,3-diphenylpropylamines which act as muscarinic receptor antagonists and are useful in the treatment of urinary incontinence and other symptoms of urinary bladder hyperactivity are known. Said compounds include N,N-diisopropyl-3-(2-hydroxy-5-methylphenyl)-3-phenylpropylamine, the (R) enantiomer of which is known as Tolterodine.
Another compound with a similar structure is 5-hydroxymethyl tolterodine, which is the main metabolite of Tolterodine (Nilvebrant et al. Pharmacol. Toxicol. 1997, 81(4), 169-172), a potent muscarinic receptor antagonist (WO 94/11337).

WO 99/058478 describes the therapeutic usefulness of phenolic esters of said main metabolite of Tolterodine, especially of isobutyric acid 2-((R)-3-N,N-diisopropylamino-1-phenylpropyl)-4-(hydroxymethyl)phenyl ester, known as Fesoterodine. Said document also describes the formation of their salts, particularly, the formation of Fesoterodine fumarate.

The synthetic processes described in document WO 94/11337 for the synthesis of Fesoterodine include the one depicted in Reaction Scheme 1. Said process comprises the formation of a dihydrocoumarin intermediate by coupling in acid medium, a transformation which takes place with a low yield; subsequently the introduction of the hydroxymethyl group takes place by means of reaction of a bromo-derivative with an organomagnesium compound, followed by reduction to hydroxyl:

The previous process has a number of drawbacks, since it is a long and expensive synthesis, which furthermore uses reagents which are difficult to handle such as organomagnesium compounds and Ni-Raney.
An alternative process for obtaining Fesoterodine is described in patent application WO 99/58478, as is shown in Reaction Schemes 2 and 3. Said process comprises the formation of a chiral amide, on which a phenylmagnesium compound is added to give rise, after the hydrolysis of the amide group, to a chiral derivative of diphenylpropanoic acid, which by means of the formation of a tertiary amide and subsequent reduction gives rise to the 3,3-diphenylpropylamino derivative intermediate (Reaction Scheme 2).

The conversion of the bromo-derivative into the hydroxymethyl group is performed through a process of metalation, carbonation and subsequent reduction of the carboxyl group obtained. Finally, the deprotection of the phenol by hydrogenation of the benzyl group and its subsequent esterification give rise to Fesoterodine (Reaction Scheme 3).

This synthesis again uses a large number of synthetic steps and uses chiral reagents, which usually have high costs, such that the industrial application of the process is difficult.
This same patent application WO 99/58478 also describes a synthesis which has fewer reaction steps to reach Fesoterodine. The process comprises a Heck reaction and the addition of phenyl-Li in a Michael type addition. However, the 2-bromoanisole derivative is a not very accessible raw material, the use of Pd reagents which make its industrialization difficult being furthermore necessary. Additionally, the global process has low or moderate yields (Reaction Scheme 4).

A different approach is described in European patent EP 1289929 B1, by means of a synthetic route in which a coupling in acid medium is initially performed, forming a dihydrocoumarin as a racemic intermediate. Said intermediate is then subjected to a stereoselective resolution process to obtain the suitable enantiomer. The latter is subsequently reduced to a lactol derivative, in which a diisoalkylamine is introduced by means of a reductive amination. Although the process is shorter, many synthesis steps are still required. In addition, the use of the aluminum tert-butoxide as a reducing agent is a considerable problem of toxicity and added cost to the process (Reaction Scheme 5).

WO 2007/138440 describes a route of synthesis through the formation of a dihydrocoumarin intermediate, by means of a reaction needing conditions of reflux in toluene and toluene/hydrochloric acid for long time periods and with a low yield (Reaction Scheme 6).

It is therefore necessary to solve the problems associated with the processes belonging to the state of the art and provide an alternative process for obtaining Fesoterodine and 3,3-diphenylpropylamine analogs which improves the synthesis of the process using more cost-effective reagents and starting materials, which furthermore allow reducing the number of steps of the synthetic route which leads to obtaining them. Advantageously, said process must be applicable at industrial scale and must provide the desired product with good yield and quality.